1. Field of the Invention
The invention relates generally to systems for guiding farm implements and more particularly to an apparatus and method using global positioning system (GPS) locations for dynamically adjusting a farm implement side-to-side to match a predetermined geographical path.
2. Description of the Prior Art
Several companies have developed farm guidance systems for close cultivation of crops. These systems are used to reduce the amount of herbicide that is needed to keep weeds out of the growing area. One such system uses a special form of three-point hitch that is allowed to move side-to-side to track drag wires or guide wheels which follow a row of plants. However, a limitation of the wire and wheel systems occurs when planting because there are no crops in the ground for the wires to follow and often there are no raised rows for the guide wheels.
Recent developments with the global position system (GPS) using real time kinematic (RTK) carrier phase or differential (DGPS) corrections have made it possible for farmers to map geographical locations within a field and then return to those locations with the accuracy that is required for cultivating and planting. Several farm guidance systems have been developed or proposed using RTK or DGPS based locations with an autopilot for steering a tractor.
However, there are limitations of existing GPS-based tractor autopilot guidance systems. First, most existing GPS-based autopilot systems operate by determining a GPS-based cross track error of the tractor and then applying a position correction to determine and correct the cross track error of the implement. Errors in the position correction can undermine the fundamental accuracy of the GPS system. More importantly, when a three-point hitch is used, the control system equations for the correction are complicated by the tendency of the implement to displace to the side opposite the direction that the tractor is steered as the tractor and implement pivot about the rear wheels of the tractor. This difficulty could be avoided with a pivoting hitch. However, the position correction would also need to track the pivot angles. Even if the position corrections could track the pivot angles, the implement would be difficult to use on a sideways sloping terrain.
Second, the weight and size of a tractor result in a slow response time for positioning a towed implement. However, it is desirable to have a fast response in order to propel the implement as fast as possible for a given acceptable cross track error.
Third, GPS-based autopilot systems are limited in a practical sense because there are a great variety of different models of tractors that have different steering requirements. Many of these tractors are very old or have steering systems that are very difficult to connect to a guidance system. Often an older tractor will require the design of a custom hydraulic controller for the tractor's power steering. The documentation for the tractor's steering system may not be available. After all is said and done the custom design may well cost more than the cost of a new tractor.
There is a need for a farming guidance system for dynamic lateral positioning of a farm implement along a geographical path where the system has a fast response time, does not require a position correction between the tractor and the implement, and does not rely on existing rows.